Why Copper is more preferable in Earthing ?

Reddish metal (Copper) is a noble metal that occurs naturally in its elemental form. And also copper is a versatile metal not only its unique properties of high electrical conductivity and high thermal conductivity but combination of these and other properties of copper.

Electrical Conductivity

Every metals has the properties of some amount of resistivity to flow of electrical current, which is why they require a power source to push the current through. Lower the resistivity, higher will be the electrical conductivity. Copper has low resistivity, so it is consider as an excellent electrical conductivity.

Corrosion Resistance

Copper is suitable for using in most of the earth electrode applications, except for acid, oxygenated ammoniac or sulphurous conditions. This is not only for high conductivity, but also for good corrosion resistance.

As per standard IS 3043, tests would be taken in wide variety of soils and it show that, the specimen of 150 mm x 25mm x 3mm of different material (copper, galvanized steel and mild steel without coating) has been buried in variety of soil for 12 years.

Copper whether it is tinned or not, is entirely satisfactory because the average loss of specimen in no case not exceed 0.2 percent per year.

Average losses of unprotected mild steel (cast iron, wrought Iron and mild steel) used in test shown as high as 2.2 percent per year.

Test showing the average loss of galvanized mild steel to be little superior to copper not greater than 0.5 percent per year.

Similarly, Copper earthing conductors, in general, need not be protected against corrosion when theyare buried in the ground if their cross-sectional area is equal to or greater than 25 mm2. So therefore, compare the average lossess among this material copper has high corrosion resistance.

Ease of joining

Copper can be readily joining by the following methods

Brazing using zinc-free brazing material with a melting point of at least 600°C;
Bolting;
Riveting and sweating; and
Explosive welding.

Mechanical Strength

Copper is both malleable and ductile metal, which means that it could be stretched to a good length without breaking or weakening it. They can be bended to fit around corners. The reddish metal stands well on this parameter.

Copper can be easily shaped into pipes, flats and wires and the copper pipes are light weighted because they have thin walls.

Temperature Tolerance

As we know the earthing conductor takes from one place to other place. When high magnitude fault current is passes through a copper conductor, its surface temperature becomes high. Not every metal sustain this temperature fluctuation but copper conductor can.

As per UL 467. the melting point of copper conductor is 1083°. so it can be used for high temperature application. So, therefore copper has high temperature tolerance than other material using in earthing

Copper can be easily combined with other metals to make alloy and it is non-magnetic and non-sparking material.

Conclusion

Though, the material of earthing conductor is selected based on its resistance value of earthing conductor, other properties also considered for the better performance.

Copper is not only having lower resistance but also combination of unique properties at economical prices.

Every grounding system need lower resistance value as possible. Lower the resistance better will be the performance of system and also easy for channelizing the fault current in to general mass earth without cause any damage.

Manufacturing Plant – Surge & Earthing Issues

Background:

A 4 decade old leading manufacturer of sheet metal components of electrical machines with sophisticated automatically operated CNC machines and robot machines wanted us to suggest the suitable surge protection devices for their factory and to study the existing earth pits on their factory premises aimed at protecting their assets.

Key Issues:

Frequent failure of electronic devices leading to downtime of machines affecting manufacturing and cost implications on replacement
Earthing was not maintained properly and the earthing conductors were found corroded at some places and loose connection was observed at some joints.
The OEM suggested providing two numbers of dedicated earthing for their machines instead of connecting to the existing earth pits.
Their vendor asked them to disconnect the other connections to the earth pit provided for their machine. They even insisted that the two earth pits provided for the same machine should not be interconnected below the ground level to ensure the dedicated earthing.

Addressing the Issues:

The audit found that protection of electronic equipment against transient surges were not provided and the need for deployment of a solution to minimize such surges entering the factory.
The earthing needs to be maintained properly and the adequate tightness of all the connections should be ensured.

Solution:

A simple solution of providing SPDs (Surge Protection Devices) at all three possible “risk areas” was recommended. This includes the Main Panel, Sub-Panel and 3 Phase drive.
Level 1 protection needs to be provided at the main panel, level 2 protection needs to be provided at the sub panel and the third stage of protection needs to be provided for individual machines.
All the earth pits shall be interconnected below the ground level for faster discharge of fault current. The references from the standards which insist the interconnection of earth pits are as follows.

IS/IEC62305 part 3 - Physical Damage to Structures and Life Hazard under clause 5.4.1 recommends the use of single integrated earth-termination system for lightning protection, power systems and telecommunication systems. Hence the earth pits of LPS shall be interconnected with the earth pits of other equipment.
IS732 - Code of Practice for Electrical Wiring Installations under clause 4.5.4.5.1 also states that “all earth electrodes associated with a building, for example, protective, functional and lightning protection, shall be interconnected“

Adequate tightness of all the joints should be ensured.
All the MS bolts & nuts in the outdoor needs to be replaced with
SS bolts & nuts.

Corrosion on Earthing Materials

Earthing is a process of giving dedicated path for the flow of high current during an abnormal condition to the ground by using earthing electrode. In order to achieve the efficiency, the earthing material should have high current carrying capacity and less corrosive.

Choice of Material of Earth Electrode

As specified by IS3043 – Code of Practice for Earthing, the conduction in an earth electrode is not ohmic but electrolytic in nature. Hence the performance of an earth electrode mainly depends on the resistivity of the soil (electrolyte) surrounding the earth electrode. The philosophy of Earthing is that “Lower the Resistivity of Soil, Better the performance of Earthing but Lower the resistivity of soil, higher will be the corrosion rate which results in shorter life time of the earthing system.

The formula provided by IS 3043 for calculating the earth electrode resistance values are as follows:

The earth electrode resistance depends on the soil resistivity and the dimensions of the earth electrodes. The ohmic resistance value of the material is not considered for the earthing design calculations and hence the kind of metal plays a role only in enhancing the life time of the earthing system and not the performance of the system. Hence it becomes extremely important to choose the right kind of electrode with appropriate dimensions mentioned in the IS standard for your earthing system for a long lasting performance, for which it is important to understand the characteristics of each metal to be used as Earth Electrode.

Most of the people believe that copper is commonly used as earth electrode because of its very low resistance value. But the actual reason for preferring copper for earth electrode is its high corrosion resistance property. IS 3043 under Clause 14.2.1 explains the thickness of different materials for being used as plate earth electrode and the details are as follows,

Under clause 14.3- Selection of metals for earth electrodes, IS 3043 explained about the corrosion rates of different materials buried in different soil conditions.

Sample dimension: 150X25X3mm

The average loss in weight of specimens of different materials buried on different soil conditions is listed below.

Cast Iron – 2.2% per year
GI – 0.5% per year
Copper - 2% per year

The corrosion rate of cast iron material is higher than that of copper and GI and hence if we go for cast iron material then the thickness of cast iron should be greater.

As few of them spoke about copper bonded rods/pipes as being earth Electrode, we as a manufacturer & services provider would like to throw some light on the same as well. Copper bonded rods are viable to be used as earthing electrodes, provided high quality standards are maintained while bonding the copper on the surface of the Mild Steel rods/pipes. Copper bonded earth electrode configurations are prescribed by the following standards.

IS 3043 – “Code of Practice for Earthing”
UL 467 – “Standard for Safety Grounding and Bonding Equipment”
BS 7430 – “Code of Practice for Earthing”
IEC 62561-2 – “Requirements for conductors and earth electrodes for Lightning Protection System”
IEC 62305-3 – “Physical Damage to Structures and Life Hazard for Protection against Lightning”
IEEE Std 80 – “IEEE Guide for Safety in AC Substation Grounding”

If high quality standards and processes are not adhered to, there is a high chance of copper getting peeled off from the electrode over a period of time leaving the mild steel rods exposed to corrosion. UL 467 an international standard followed by various countries as a benchmark for producing high quality copper bonded rods prescribes a strict quality process of manufacturing such rods to ensure the life of the electrode does not diminish earlier than the expected time frame. Some of the tests carried out by the UL as quality standards are

Adherence of coating test.
Bending test.
Thickness of coating test.

Conclusion:

As the resistance of an earth electrode doesn’t depend on the material, the selection of the material mainly depends on the local site conditions, life expectancy, and financial feasibility of the particular project.

Current Carrying Capacity Of Different Materials

The Earth electrodes should be designed to withstand the fault current without any physical damages. The fault current carrying capacity of a material mainly depends on cross sectional area of that particular material. In this article, we shall discuss about the current carrying capacity of pipes and strips of different dimensions and different materials as specified by IS 3043:2018.

In general, the amount of electric current flowing per unit cross sectional area of a material is called current density of a material. The current density of the earth electrode can also be calculated by the formula specified in IS 3043,

Measurement of Earth Resistivity

The earth resistivity extremely varies between 1 to 10,000-ohm meters and in most situations, the soil resistivity is found to be non-uniform. Variation of the soil resistivity in depths is more predominant as compared to the variation with same in the horizontal distance and this happens due to stratification of earth layer.

Measurement of earth resistivity at the sites will reveal the characteristics of soil whether it is homogenous or nonuniform.

Because of moisture content in the soil, there is a wide range of variation in soil resistivity so it is advisable to take the testing during the dry season for considerable values.

Test Location

In substation or generating station, at least eight test directions shall be chosen from the center of the station to cover the entire site and it will be increased depending upon how larger the site will be.

For the transmission line, the measurements shall be taken along the direction of the line throughout the length approximately once in every 4 kilometers.

Principle of Tests

Wenner’s four-electrode method is widely used for measuring soil resistivity in site location.

In this method, four electrodes are driven in a straight line at equal intervals into the earth. A current is passed to the outer two electrodes and the earth at the same time, the voltage will be observed between the inner two electrodes.

The resistivity is going to be proportional to the ratio of the voltage to current accordingly.

If the depth of burial of the electrodes in the ground is negligible compared to the spacing between the electrodes, then the resistivity value is

Four terminal megger earth tester is the most frequent method used for measuring soil resistivity, which comprises the current source and meter in a single instrument and directly reads the resistance.

The modified equation for soil resistivity is below,

Where,

ρ - Resistivity of soil in ohmmeter

s = Distance between the successive electrodes in meters,

R = Megger reading in ohms.

Test Procedure

By Wenner’s four-point method, four electrodes are driven into a straight line at equal intervals in the chosen direction of the selected test site.

The depth of the electrode in the ground shall be of the order of 10 to 15cm, the megger is placed on the steady and approximately level base, and the four electrodes connected to the megger terminal.

The readings are taken while turning the crank is about 135 rev/min. The measuring values are substituted in the equation to find out the soil resistivity.

Correction of Potential Electrode Resistance

In case, the resistance of the potential electrode (inner electrode) is comparatively high, a correction of the test
the result would be necessary on its value.

The corrected value of the earth resistivity would be:

Where,
ρ = Corrected value of earth resistivity,

ρ' = Uncorrected value of soil resistivity,

Rp = Megger value for corrected connection.

Rv = Resistance of the voltage circuit of the instrument used to obtain R (as indicated inside the scale cover of the meter)

For this purpose, the instrument is connected to the electrode as,

Impact of Soil Resistivity on Grounding

The Earthing Resistance of any electrode is made up of the following factors:

• Resistance of the (metal) electrode,

• Contact resistance range between the electrode and the soil

• Resistance of the soil from the electrode surface outward in the geometry (which is set up for the flow of
current outward from the electrode) to infinite earth.

Among these three, Soil Resistivity plays a vital role in the performance of Earthing

Soil Resistivity:

The Resistance to the earth of a particular electrode depends upon the electrical resistivity of the soil where it is installed. Soil resistivity has a direct effect on the resistivity of the earthing system and it is an indication of a
given soil’s ability to carry electric current to flow.

Earth conductivity depends on some of the contributory factors on given below

✓ Electrolytic in nature,

✓ Moisture content of soil,

✓ Chemical composition,

✓ Concentration of soil dissolved in water,

✓ Grain size and distribution,

✓ Closeness of packaging.

Hint: Many of these factors may vary locally, occasionally, and seasonally as well.

Effect of Temperature on Earth Resistivity

• The temperature coefficient of resistivity for soil will be negative but is negligible for temperatures
above freezing point.

• At about the temperature of 20°C, the resistivity changes about 9 percent for every degree Celsius.

• Below 0°C the water in the soil starts to freeze itself and generates an enormous increase in the
temperature coefficient values.

• As the temperature becomes lower, the resistivity increases drastically. Therefore, we recommended the
earth electrode should be well below the frost line.

Effect of moisture content on Earth Resistivity

▪ The resistance of the earthing electrode is depends upon the soil resistivity of particular location.

▪ Moisture content is one of the controlling factors in measuring soil resistivity.

▪ It is advisable to conduct earth resistivity tests during the dry season.

▪ As per standards, if moisture content in soil is below 20%, the resistivity increases very abruptly and if it
is above 20% there will be little impact on soil resistivity.

▪ The normal moisture content of soils ranges from 10% (in dry season) to 35% (in wet seasons) and an
approximate range may accounts to 16 to 18 percent.

Effect of salt content on Earth Resistivity

❖ Pure water is bad conductor of electricity. Resistivity of soil depends on the resistivity of water which in
fact depends on the amount and nature of salts dissolved in it.

❖ The Resistivity of the soil will be variable from various ranges to 5% of salt content and a further
increase in salt in soil gives no appreciable benefits in improving resistivity.

❖ Small quantity of salt with moisture content reduces soil resistivity in large value but it corrodes the
earthing electrode which affects the performance of earthing.

Impact of Soil Resistivity on Grounding

▪ When current flows from a ground electrode into the surrounding soil, it is described as flowing through
a series of concentric shells of corresponding increasing diameter.

▪ Each and every concentric shells having greater area to dissipate current into the soil, so resistance
become less.

▪ When the current dissipation is in larger manner, the current density becomes smaller that the resistance
will be very less.

▪ But in high resistive area, the above condition is reverse which leads to higher touch and step potential
and also damage the nearby electrical appliance.

Electrical Conductivity

Corrosion Resistance

Ease of joining

Mechanical Strength

Temperature Tolerance

Conclusion

Background:

Key Issues:

Addressing the Issues:

Solution:

Test Location

Principle of Tests

Test Procedure

Correction of Potential Electrode Resistance

Soil Resistivity:

Effect of Temperature on Earth Resistivity

Effect of moisture content on Earth Resistivity

Effect of salt content on Earth Resistivity

Impact of Soil Resistivity on Grounding

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